The present disclosure relates to a curl eliminating device for eliminating a curl of a recording medium such as a paper sheet and to an image forming apparatus including the curl eliminating device.
Conventionally, in an electrophotographic type or an inkjet recording type image forming apparatus such as a copier or a printer, a double-sided printing method is widely used, in which the paper sheet after forming an image on one side of the paper sheet is not discharged but is conveyed again to an image forming unit so that an image is formed on the other side too. The double-sided printing type image forming apparatus is equipped with a double-sided circulation transport path for reversing the paper sheet after an image is formed on one side thereof, so as to reconvey the reversed paper sheet to the image forming unit.
As to the image forming apparatus of the double-sided printing type described above, it is common to switch back the paper sheet in the double-sided circulation transport path, to stop the paper sheet temporarily on a reverse tray, and then to reconvey the paper sheet from the rear end so that the front and back sides of the paper sheet are reversed. For this reason, when the paper sheet passes through the double-sided circulation transport path having a U-shaped curve, a curl (curling) may occur to the paper sheet. Main causes of the curl include contraction of a resin component in toner due to thermal fixing in an electrophotographic image forming apparatus, swelling of the paper sheet due to impregnation of ink in an inkjet image forming apparatus, and a moisture control function of the paper sheet depending on temperature and humidity conditions.
For instance, when continuously printing on both sides of the paper sheets, in order to improve print efficiency, during printing on a second side, the next paper sheet having the first side printed is set on standby in the double-sided circulation transport path. In this case, curl may occur to the paper sheet stopped at a bent part of the double-sided circulation transport path. In particular, as need for bookbinding by on-demand printing has been increased, frequency of using thick paper sheets having basis weight of 300 g/m2 as cover papers of booklets has been increased. Such thick paper sheets tend to occur curls, which is hardly straightened.
Accordingly, there are proposed various curl eliminating devices for eliminating curl of a paper sheet. For instance, there is known a curl correction device for correcting curl of a paper sheet by making the paper sheet pass through a nip portion between a pressure roller and a belt. It is also known to adjust pressing amount of the pressure roller to the belt by using a cam.
In addition, there is known a curl eliminating device in which first and third rollers as compressive soft rollers are pressed to contact with both sides of a second roller as a non-compressive hard roller, and a nip portion for the sheet to pass through is determined in accordance with direction of the curl of the sheet.
Further, there is known a decurl device including a decurl part capable of moving in the direction perpendicular to the conveying direction of continuous paper, in which the decurl part is moved opposite to the curl direction of the continuous paper so that a curled part is straightened by a decurl bar for removing the curl.